The invention relates to a method in a frequency converter provided with a voltage intermediate circuit and to a frequency converter. In particular, the invention relates to the operation of a frequency converter in connection with interference in network voltage.
A frequency converter is a device which is generally used in connection with motors and generators. In motor use, the frequency converter is supplied by the network and it generates voltage for the motor for controlling it in the desired manner. In direct generator use, i.e. in a use where the power generated by the generator is transferred through the frequency converter, the frequency converter may convert the voltage generated by the generator into voltage that can be supplied to the network.
Many existing frequency converters create an electric model of the machine to be controlled. This model provides the frequency converter with as accurate information as possible on the electric state of the machine and on its magnetic fluxes in addition to currents and voltages. By means of electric models, the torque generated by the machine, for example, can be adjusted in a very rapid time domain. The network converter part of the frequency converter may create an electric model of the network to be supplied in a corresponding manner.
When the network voltage decreases or disappears, the network converter part of the frequency converter can transfer only a very small amount of the effective power produced by the generator to the network or no effective power at all. As a result of this, the network converter current increases up to a set current limit and, at the same time, the voltage of the DC intermediate circuit rises. Thus the generator produces power that cannot be transferred to the network in its entirety.
The simplest solution for protecting a frequency converter is to disconnect the generator from the inverter of the frequency converter when the intermediate circuit voltage increases too much. Such a solution is not, however, cost effective since the connecting and disconnecting are slow procedures, requiring re-start of the whole system.
Earlier, the rise in the intermediate circuit voltage has been limited employing various brake choppers or crowbar arrangements, where the produced power is used in resistor resistances. A disadvantage associated with such solutions is that if it has been necessary to end the modulation of the frequency converter, the generator start-up and synchronization cause a generator delay. The ending of modulation results in a loss of accurate information on the electric state of the generator and network, and consequently, when the network normalizes, the frequency converter has to synchronize with the generator and the network to be supplied.